1. Field of the Invention
This invention relates to an apparatus and process for feeding hypochlorite solution form a source of the solution to a device for removing impurities from water in a water treatment system.
2. Description of the Prior Art
In many water treatment systems, particularly oilfield water systems, filters must be treated with biocides to control bacterial contamination. Bacterial contamination of filter media, if left uncontrolled, can cause bacterial contamination and subsequent corrosion of downstream equipment and wells, produce high hydrogen sulfide (H.sub.2 S) levels in the filter effluent, and cause decreased filtration efficiency due to plugging of the media with biomass.
Many types of biocides have been employed in treating oilfield filtration systems, including: acrolein, glutaraldehyde, quaternary amines, gaseous chlorine, chlorine dioxide and sodium hypochlorite solution. While all of these biocides are effective in killing sulfate-reducing bacteria and other undesirable bacteria, only acrolein, chlorine, chlorine dioxide and hypochlorite react directly with hydrogen sulfide. Furthermore, only chlorine, chlorine dioxide and hypochlorite react to convert ferrous iron to filterable ferrix oxide. Therefore, the best results in terms of oilfield water filter performance and filter effluent water quality are obtained when using either chlorine, chlorine dioxide or hypochlorite solutions to treat oilfield filters.
Hypochlorite solutions are typically applied to the filter in one of three ways: continuous addition at a constant rate to the filter inlet; intermittent "shot" feeding to the filter inlet; and addition to the filter backwash water during the backwash cycle. However, each of these treatment methods suffers from its own disadvantage. For example, continuous addition of a fixed concentration of biocide does not account for the fact that bacterial contamination levels within a filter vessel change as a function of the filtration cycle. Bacterial populations are usually at their lowest levels immediately following a backwash cycle. However, as a filter progresses through its service cycle, debris in the form of oil, sediment and biomass accumulates and bacterial population levels increase. Because of this phenomenon, a fixed, continuous biocide feed-rate results in a wasteful overfeed at the beginning of a service cycle and an ineffective underfeed at the end of a service cycle.
Intermittent "shot" feeding allows a relatively high concentration of biocide to be fed during the period of the shot. This provides good disinfection of the filters during the shot and for a period of time afterward. However, as noted above, biomass and other debris accumulate in the filter media continuously throughout a service cycle. This promotes a rapid proliferation of bacteria whenever biocide is not present. Also, H.sub.2 S and dissolved ferrous iron are typically present in the influent water to oilfield water filters. These undesirable contaminants pass through the filter whenever oxidizing biocide is not being fed.
Addition of biocide during the filter backwash typically accomplishes good disinfection of the filter media. However, as noted above, bacterial recontamination occurs fairly rapidly and there is no effective removal of H.sub.2 S and ferrous iron during the service cycle.